Afghan leader Ashraf Ghani will meet with US President Barack Obama to discuss the pace of American troop withdrawals from the troubled country.

US President Barack Obama will meet with Afghan leader Ashraf Ghani today to discuss the pace of American troop withdrawals from the troubled country, amid reports of fresh violence and a American drone stroke on militants in the region.

The US-led NATO combat mission to Afghanistan officially ended on December 31, 2014, at which point responsibility for security was passed to local forces. However, 10,000 American personnel remain, under the terms of a last-minute deal that allowed a military presence to stay on past the deadline. Almost half of the troops are currently scheduled to be pulled out by the end of this year and the force completely withdrawn by 2017.

Yet insurgent groups, particularly the Taliban, are still proving difficult to deal with and inflicting heavy casualties on Afghan security forces. So Ghani has asked for a degree of flexibility in the US withdrawal timetable. It's a request the White House is "actively considering," US Secretary of State John Kerry said on Monday after talks at Camp David to discuss future relations between the two countries.

The US also pledged to fund Afghan forces until the end of 2017 in an attempt to boost stability, while Kerry added that as much as $800 million would be dedicated to a newly established development partnership designed to boost the sustainability and economic transparency of Afghan institutions.

Ghani declined to specify how many American soldiers he wished to remain in the country, saying it was a decision for Obama. In a visit to the Pentagon on Monday he also thanked US personnel who had served in Afghanistan for the sacrifices they had made for his country. More than 2,300 Americans have died there since a US-led coalition toppled the hardline Islamist Taliban government in 2001.

The talks come as unknown gunmen killed at least 13 people in the east of the country. The attack took place in the early hours of Tuesday in the Sayad Abad distrcit of Wardak province when masked militants fired on three vehicles, including a bus from Kabul, according to remarks made by a provincial spokesperson carried in the Associated Press

It was the third time that bus passengers have been attacked in the past month. The previous incidents saw members of the country's minority Hazara group targeted. Hazara are overwhelmingly Shia and were persecuted under the Taliban's hardline Sunni rule.

Wardak has a strong Taliban presence and the group has targeted security forces there in the past. However neither it, not any other insurgent organization, claimed responsibility for the attack.

Meanwhile, an American drone strike reportedly killed at least nine Pakistani militants in the Afghan province of Nangarhar early Tuesday.

The attack took place close to a Pakistani border region where militants have been clashing with government forces. Pakistani intelligence officials told Reuters that the dead had been fighters with the Pakistani Taliban and allied Lashkar-e-Islam.

The existing deal for US troops to stay longer in Afghanistan was only signed in December after Ghani was sworn into office at the head of a unity government with his electoral rival Abdullah Abdullah as chief executive (a position roughly analogous to an executive prime minister) after months of political deadlock.

Ghani's predecessor Hamid Karzai had refused to ratify the deal, even though US officials had signaled that this would result in a full American withdrawal. It was one of many sources of contention between the US administration and Karzai, who frequently lambasted the US for what he described as interference in Afghan affairs.

A similar agreement allowing a NATO force to stay in Afghanistan past December was also signed at the end of 2014 and around 2,000 personnel are currently contributed by non-US NATO states to a combined international force, which provides training and help to local soldiers. NATO troops have played an important role in Afghanistan's security for well over a decade and during peak deployment in 2012, there were more than 130,000 based in the country.

Ghani previously stressed that, despite a foreign military presence remaining in the country, Afghanistan would retain control over its airspace, that the international force would only be used based on decisions made by the Afghan government, and that international troops would not be able to enter mosques or other holy sites.

Meanwhile, Afghan security forces are shrinking due to casualties and desertion, according to a report released earlier this month by American Afghan watchdog the Special Inspector General for Afghanistan Reconstruction (SIGAR). The Afghan army shrunk from 184,839 to 169,203 in the 12 months from the third quarter of 2013, and while police numbers were reportedly up 3,122 to 156,439 in 2014, SIGAR said some may have been counted twice. It added that the actual figure could in fact be down 2,045 over just three months.

The report concluded that heavy fighting with insurgents was part of the cause. "The ANA [Afghan National Army] continues to suffer serious combat losses," it said. "Between October 2013 and September 2014, more than 1,300 ANA personnel were killed in action and 6,200 were wounded in action."